In radio communication, it is difficult to directly transmit a DC signal or low-frequency signal. In general, such a signal is transmitted by performing information modulation to a high-frequency carrier. More specifically, the transmitting side modulates a carrier by a signal wave to be transmitted, and transmits the modulated wave. The receiving side demodulates the received modulated wave to extract the signal wave from the carrier and obtain transmitted data (see, e.g., reference: Hideo Ohba & Hideki Sagesaka, “Radio Communication Device”, Nihon Riko Shuppankai, pp. 141-265, ISBN 4-89019-136-4).
FIG. 32 shows an example of the configuration of a conventional radio communication system. In FIG. 32, a VCO (Voltage Controlled Oscillator) 1001 generates a carrier on the transmitting side. The carrier is modulated by multiplying the carrier by a baseband signal IN to be transmitted by a multiplier 1002. The modulated wave is amplified by a PA (Power Amplifier) 1005, and transmitted from a transmission antenna 1006. On the receiving side, the modulated wave received by a reception antenna 1011 is amplified by an LNA (Low Noise Amplifier) 1012, and the image component is rejected by an image rejection filter 1013. The modulated wave from which the image component is rejected is down-converted by multiplying by a multiplier 1017 the modulated wave by a carrier generated by a VCO 1016. The modulated wave passes through a channel selection filter 1018, and is converted by a detector 1019 into the transmitted baseband signal. FIG. 32 illustrates an example of radio communication by PSK (Phase Shift Keying). Also in another radio communication method, radio communication is generally done by generating a carrier and modulating/demodulating the carrier.
As a system which communicates without using any carrier, there has been proposed a radio communication system using the UWB (Ultra WideBand) technique (e.g., PCT(WO) 2003-529273 (WO 01/073965), and PCT(WO) 2003-535552 (WO 01/093441)). The UWB transmitter sends pulses of a very short time base 1,000,000,000 times or more per second in a very wide frequency band of several GHz. The receiver receives a sequence of pulses sent from the transmitter, and converts the pulses into data.
As described above, the conventional radio communication system shown in FIG. 32 generates a carrier in transmission and reception, modulates/demodulates the carrier, and communicates by radio. For this reason, the radio communication system requires a circuit for generating a carrier and a circuit for modulating/demodulating a carrier. The radio communication system becomes complicated, the scales of the transmitter and receiver which form the radio communication system and the hardware size increase, and the cost and power consumption of the radio communication system rise.
A radio communication system using UWB also requires a circuit for generating a monocycle waveform or Gaussian monopulse of a short time width. The design becomes difficult because a high-frequency analog circuit technique is necessary to form such a circuit. The scales of the transmitter and receiver which form the radio communication system and the hardware size increase, and the cost and power consumption of the radio communication system rise.